Optical magnetometer using a laser coupled to a magneto-optical medium

ABSTRACT

An optical magnetometer for measuring the component h of a magnetic field in a direction x by the Faraday effect. A semiconductor laser produces two laser beams which are directed from opposite faces of the laser in opposite directions along two fiber optic cables. The opposite beams of light enter a magneto-optical medium and then continue via the opposite fiber optic cable to return to the laser where the two light beams are recoupled. An alternating magnetic field at frequency f and a compensation field are applied to the magneto-optical medium in a direction parallel to the direction x to produce the Faraday effect. The laser intensity of the recoupled light beams is proportional to the magnetic field to be measured. A photodetector samples portions of the recoupled light beam and provides a current i d  which is proportional to the recoupled beam intensity. The current i d  is filtered at frequencies f, for compensating the field h to be measured, and 2f. The magneto-optical medium used is preferably formed by a substituted YIG ferrimagnetic layer, grown by epitaxy on a non-magnetic garnet.

BACKGROUND OF THE INVENTION

The present invention relates to an optical magnetometer able to detectweak magnetic fields of approximately 10⁻⁵ A/m.

It is known to measure the rotation of the polarization plane of linearpolarized light supplied by a laser on traversing a magnetoopticalmedium subject to a magnetic field. This rotation, known as a Faradayrotation, is a function of the value of the component of the magneticfield parallel to the propagation direction, which makes it possible toobtain a field measurement by measuring said rotation.

SUMMARY OF THE INVENTION

The present invention leads to improvements in the performance of theprior art optical magnetometers by recoupling the optical beam which hastraversed the medium in the laser source cavity.

The invention specifically relates to an optical magnetometer formeasuring the component h of a magnetic field in a direction x,comprising a laser source supplying a linear polarized light to amagnetooptical medium having the Faraday effect, which it traversesparallel to the direction x, wherein the optical beams leaving by thetwo faces f₁ and f₂ of the laser traverse the magnetooptical medium inopposite directions and following said passage, these two beams arerecoupled in the laser, the beam leaving by face f₁ being recoupled byface f₂ and the beam leaving by face f₂ being recoupled by face f₁,whilst the magnetic field is measured on the basis of the current i_(d)supplied by a photodetector receiving part of the optical power of thelaser.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention can be gathered from thefollowing description with reference to the drawings, which show:

FIG. 1 is a circuit diagram of the magnetometer according to theinvention;

FIG. 2 a diagram showing a laser with recoupling by the reflected light;

FIG. 3 a diagram of an optical magnetometer with its processing circuitsaccording to the invention;

FIG. 4 the coupling of an optical fiber with a ferrimagnetic materiallayer deposited by epitaxy;

FIG. 5 the diagram of a strained optical fiber phase shifter.

DETAILED DESCRIPTION OF THE INVENTION

The magnetometer according to the invention combines a magnetoopticalmedium and a laser, which is preferably a semiconductor laser. Itinvolves the combination of the magnetic circular birefringence effect,which occurs when light waves traverse a magnetooptical medium and whichis responsible for the Faraday effect and the modulation effect of thelaser emission by an optical feedback exerted on the laser cavity. Themeasurement of the magnetic field is deduced from the measurement of thecurrent intensity at the output of a photodetector intercepting theresulting laser emission.

The general block diagram of the magnetometer is shown in FIG. 1. Itcomprises a semiconductor laser source 10 of the type having a polarizedjunction or "laser diode". The two light beams 101, 102 emerging fromthe two opposite faces f₁ and f₂ of the laser diode, pass through amagnetooptical medium 11 in opposite directions. Each beam is recoupledinto the optical cavity of the laser diode in such a way that it entersit by the opposite face to that used on emission. Thus, the two beamsinterfere in the optical cavity with the stored energy. Preferably, andas shown in the drawing, the optical beams propagate in an opticalfiber.

The light emitted by the laser and whose injection current is higherthan the threshold current, is essentially polarized in a linear mannerin accordance with the TE mode and is monomode.

In the presence of a magnetic field H, the magnetooptical medium 11rotates the polarization plane of linear light by a magnetic circularbirefringence effect or Faraday effect. As the Faraday rotation isindependent of the light propagation direction, it takes place in thesame direction for both beams 101 and 102. The magnetooptical medium ischosen so that the Faraday rotation angle is proportional to thecomponent h of the magnetic field H, parallel to the propagationdirection x of the light in the medium. Part of the resulting laseremission 103 is intercepted by a photodetector 12 in order to obtain themeasurement signal.

For each beam recoupled into the cavity, the Faraday rotation leads to amodification of the amplitude of the associated luminous vibration andit undergoes a phase shift due to the traversed optical media. A doubleoptical feedback is exerted on the cavity and leads to a modulation ofthe intensity supplied by the laser diode, which is a function of thedirection of the polarization plane. This intensity is a function of theFaraday rotation angle and of the phase shift, whose values areidentical for both beams.

An alternating magnetic field h_(o) sin 2πft is produced by the current,traversing a coil, supplied by a generator 15. The direction of thisalternating magnetic field is parallel to the light propagation and issuperimposed on the field h to be measured. The Faraday rotation angle θis consequently dependent on time in accordance with the relationθ=A(h+h_(o) sin 2πft), in which A is a constant dependent on the type ofmagnetooptical medium used and on its length.

The current i_(d) detected at the output of photodetector 12 is filteredaround the frequency f by a filter 14 and around the frequency 2f by afilter 17. At the output of filter 14 is obtained a signal B, whoseamplitude is proportional to h, in accordance with the expression K₁ Ahsin Ωt, in which K₁ is a proportionality factor dependent on the productAh_(o) and on the phase shift φ of the light wave along the feedbackloop, between the input and output of the laser cavity (Ω=2πf). At theoutput of filter 17 is obtained a signal, whose amplitude is constant inaccordance with K₂ cos 2Ωt, in which K₂ is dependent on Ah_(o) and φ.

It is known that if a mirror 20 (FIG. 2) is placed perpendicular to thebeam of a laser 21, the power of the laser fluctuates as a function ofthe distance d of mirror 20 from laser 21. This power measured bydetector 22 passes through a maximum when the reflected amplitude is inphase with that of the laser, and through a minimum when these twoamplitudes are in phase opposition. To optimize the arrangement of FIG.1, it is consequently necessary to optimize phase φ.

On referring to FIG. 1, an optical phase shifter 13 is introduced intothe feedback loop in order to adjust the value of φ by control 18, so asto make maximum the amplitude of the output signal of filter 17, i.e.K₂, read on indicator 19. This optimization makes it possible to alsomake factor K₁ maximum as a function of the product Ah_(o), A beingfixed for a given magnetooptical material, as will be seen hereinafter.

Thus, any variation of the phase shift φ under the effect of externalparameters, particularly the temperature, has no influence on themeasurement of h, because the phase shift φ can be permanently adjustedduring the measurement. Moreover, the device is consequently at itsmaximum sensitivity.

The known techniques of guided optics are used for producing themagnetometer according to the invention. In particular, the light ispropagated from one face to the other of the laser cavity by means of anoptical fiber. This optical fiber is chosen in such a way as to retainthe light polarization, as is for example described in an article by R.E. WAGNER et al, Electronics Letters, Vol. 17, No. 5 of 5.3.1981,pp.177/8.

The preferred embodiment of the invention is shown in FIG. 3, in whichan optical fiber 31 maintaining the polarization of the light is usedfor connecting the two output faces of a laser diode 30 to amagnetooptical medium 32. The wavelength λ of the laser diode is chosenin such a way as to be matched to the magnetooptical medium used(optimum absorption/Faraday rotation ratio).

According to this preferred embodiment of the invention, themagnetooptical medium 32 is a ferrimagnetic material having spontaneousmagnetization. In the case of such materials, it is known that byapplying a so-called saturation magnetic field H_(S) orienting all themagnetic domains in the same direction, rotation θ is given by: θ=(θ_(o)/H_(S))Lh, in which θ_(o) is the specific Faraday rotation, obtained fora field ≧H_(S), L being the length of the material traversed.

Thus, the rotation θ is proportional to the field h, and the value A,indicated hereinbefore, is equal to (θ_(o) /H_(s)). By making an analogywith materials having induced magnetization for which the rotation isproportional to the following field CLh, C being the Verdet's constantof the material, the ratio (θ_(o) /H_(S)) represents an "apparent"Verdet's constant C_(A). The latter has a much higher value than C,namely approximately 10⁶ times. Thus, it is of interest, for reducinglength L, to use a material having a spontaneous magnetization.

According to the preferred embodiment of the invention, the materialused for forming the magnetooptical medium has a high "apparent"Verdet's constant, easy magnetization in one plane and a weak saturationfield.

The formation of the magnetooptical medium is diagrammatically shown insection in FIG. 4. It is advantageously constituted by a gallium andgadolinium substituted yttrium-iron garnet layer 41. This layer isobtained by epitaxy on a gadolinium-gallium garnet substrate 42, calledGGG. The coupling between the layer and the fiber is directly obtained.

The saturation field H_(S) is applied in the plane of the layer 41perpendicular to the light propagation direction, using a permanentmagnet 33 or a solenoid. To obtain a single-domain layer, it is merelynecessary to have a weak intensity field of approximately 80 A/m.

The alternating exciting magnetic field h_(o) sin 2πft is applied to themagnetooptical medium by means of a solenoid 35 which surrounds thelatter, its axis coinciding with the light propagation direction. Thissolenoid is supplied by generator 36 at frequency f.

This device comprises a photodiode 37, which intercepts part of thelight leaving by one of the faces of the laser diode, either directly,or by means of a guided optics directional coupler, 38, placed againstone of the faces of the laser diode. The electrical intensity i_(d)supplied by the photodiode is filtered around frequency f anddemodulated by a synchronous detection amplifier 39 receiving thereference current from generator 36. This intensity is also filteredaround frequency 2f in 310. The component of i_(d) at frequency f istransmitted into a group of circuits 300 supplying the value of thefield h, to be measured by a zero method. On magnetic field h_(o) issuperimposed a magnetic compensation field, by injecting a direct orvery low frequency current i_(c) into coil 35, said current being addedto the exciting current i_(E) in adder 301. It is obtained at the outputof a differential amplifier 302, which receives the demodulatedcomponent of i_(d) at frequency f. The measurement of the field isdirectly obtained on indicator 303, which symbolizes any dataacquisition system used in such cases.

The device incorporates a variable optical phase shifter 34 shown inFIG. 5. This phase shifter is realized by means of a piezo-electricsupport 50 in the form of a wafer, to which is bonded optical fiber 31in such a way that it is rendered integral with the support. A variablevoltage V is applied to the terminals of the wafer and the resultingoptical path variations make it possible to adjust the phase shift.Prior to the measurements, the phase shifter is controlled at 312, so asto make maximum the amplitude of the signal from filter 310, indicatedby circuit 311. During the measurement, circuit 311 automaticallycontrols the phase shifter, so as to maintain said amplitude maximum.According to a variant of the invention, the magnetooptical medium isconstituted by the transmission optical fiber. As the Verdet's constantof the materials forming the fibers is generally low, the fiber has aconsiderable length, namely several hundred meters, in order to obtainan adequate sensitivity. Thus, the fiber is wound in the form of a flatcoil, in order to form turns, which are placed within the exciting coil,the transverse saturation field H_(S) then no longer being necessary.

The aforementioned device makes it possible to detect relativevariations of component i_(d) of approximately 10⁻⁶.

The laser emission wavelength is, for example, equal to 1.2 μm and thecore diameter of the optical fiber is equal to approximately 5 μm.

The following characteristics are obtained with a magnetooptical mediumformed by a thin GaGdYIG layer:

    θ.sub.o =4.3 10.sup.2 rad/m

    H.sub.S =80A/m

    L=1 cm

    h.sub.o =24A/m

    h minimum detectable=10.sup.-5 A/m.

The following characteristics are obtained with a magnetooptical mediumconstituted by a silica fiber:

    Verdet's constant C=1.1×10.sup.-6 rad/m(A/m)

    L=1000 m

    h.sub.o =130A/m

    h minimum detectable=3×10.sup.-4 A/m.

What is claimed is:
 1. An optical magnetometer for measuring thecomponent h of a magnetic field in a direction x, comprising:laser meanshaving first and second opposite faces, for supplying linear polarizedlight through said faces; first polarization-retaining opticaltransmission means, coupled to said first face, for carrying said light;magneto-optical means, having a first coupling end and a second couplingend coupled to said first transmission means and arranged so that lightentering said magneto-optical means travels in a direction parallel tox, for producing a Faraday effect on said light; means for inducing insaid magneto-optical means an alternating magnetic field superimposed onsaid component h parallel to said direction x and having a frequency f;second polarization-retaining optical transmission means, coupled tosaid second face and said first coupling end, for carrying said light tocause said light from said first and second faces to travel in oppositedirections and return to said laser means at a face opposite the facesaid light was transmitted through, to cause said light traveling inopposite directions to be recoupled at said laser means; opticaltransmission means, coupled to said second face, for receiving a portionof said recoupled light; photodetector means, coupled to said opticaltransmission means, for producing a signal i_(d) corresponding to saidrecoupled light; and first filter means, coupled to said photodetectormeans, for filtering said signal i_(d) at said frequency f and providingan output signal proportional to said component h.
 2. An opticalmagnetometer according to claim 1, further including:a controllableoptical phase shifter inserted onto one of said first and secondtransmission means; second filter means, coupled to said photodetectormeans, for filtering said signal i_(d) at a frequency twice saidfrequency f and delivering to said controllable phase shifter a firstfeedback control signal for adjusting a value of said signal i_(d) to amaximum.
 3. An optical magnetometer according to claim 2, wherein saidphase shifter includes a piezoelectric wafer onto which is bonded saidone of said first and second transmission means, and means for applyingto said wafer said feedback control signal.
 4. An optical magnetometeraccording to claim 3, further including control means, coupled to saidfirst filter means and to said inducing means, for receiving said outputsignal and delivering to said inducing means a second feedback controlsignal for controlling said inducing means to cause said inducing meansto further induce a magnetic compensation field which zeroes saidcomponent h, whereby said second feedback control signal measures thevalue of said component h.
 5. An optical magnetometer according to claim4, wherein said inducing means includes a coil surrounding saidmagneto-optical means and having an axis parallel to said direction x, agenerator supplying an alternating current having said frequency f, andwherein said control means includes a synchronous detection amplifierreceiving said signal i_(d) and said alternating current, a differentialamplifier connected to said amplifier and delivering said secondfeedback control signal, and an adder receiving said alternating currentand said second feedback control signal and supplying said coil.
 6. Amagnetometer according to claim 5, wherein said magneto-optical meansincludes a thin layer of yttrium-iron garnet with gallium and gadoliniumsubstitution, grown by epitaxy on the gadolinium gallium garnet.
 7. Anoptical magnetometer according to claim 5, wherein said magneto-opticalmeans includes a layer of ferrimagnetic material having spontaneousmagnetization, and a permanent magnet producing a saturation magneticfield perpendicular to said direction x in a plane of said ferrimagneticmaterial having easy spontaneous magnetization.
 8. A magnetometeraccording to claim 1, wherein said magneto-optical means includes aparamagnetic material with induced magnetization.
 9. A magnetometeraccording to claim 8, wherein said magneto-optical means includes aparamagnetic optical fiber.
 10. A magnetometer according to claim 1,wherein said laser is a semiconductor laser emitting a wavelength in atransparency range of said magneto-optical means and having an optimumabsorption/Faraday rotation ratio.